Note 6 - Fair Value Measurements - Assets and Liabilities Carried at Fair Value on a Recurring Basis (Details) - Reported Value Measurement [Member] - Fair Value, Recurring [Member] - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020
Cash equivalents	$ 35,613	$ 19,394
Marketable securities	10,191	35,195
Fair Value, Inputs, Level 1 [Member]
Cash equivalents	35,613	19,394
Marketable securities	10,191	35,195
Fair Value, Inputs, Level 2 [Member]
Cash equivalents
Marketable securities
Fair Value, Inputs, Level 3 [Member]
Cash equivalents
Marketable securities